Scientists
ringing alarm bells about the melting of Antarctica have focused most
of their attention, so far, on the smaller West Antarctic ice sheet,
which is grounded deep below sea level and highly exposed to the
influence of warming seas. But new research published in the journal
Nature Wednesday reaffirms that there’s a possibly even bigger —
if slower moving — threat in the much larger ice mass of East
Antarctica.

The
Totten Glacier holds back more ice than any other in East Antarctica,
which is itself the biggest ice mass in the world by far. Totten,
which lies due south of Western Australia, currently reaches the
ocean in the form of a floating shelf of ice that’s 90 miles by 22
miles in area. But the entire region, or what scientists call a
“catchment,” that could someday flow into the sea in this area is
over 200,000 square miles in size — bigger than California.

Moreover,
in some areas that ice is close to 2.5 miles thick, with over a mile
of that vertical extent reaching below the surface of the ocean. It’s
the very definition of vast.

Warmer
waters in this area could, therefore, ultimately be even more
damaging than what’s happening in West Antarctica — and the total
amount of ice that could someday be lost would raise sea levels by as
much as 13 feet.

“This
is not the first part of East Antarctica that’s likely to show a
multi-meter response to climate change,” said Alan Aitken, the new
study’s lead author and a researcher with the University of Western
Australia in Perth. “But it might be the biggest in the end,
because it’s continually unstable as you go towards the interior of
the continent.”

The
research — which found that Totten Glacier, and the ice system of
which it is part, has retreated many times in the past and contains
several key zones of instability — was conducted in collaboration
with a team of international scientists from the United States,
Australia, New Zealand and the United Kingdom. A press statement
about the study from the U.S. group, based at the University of Texas
at Austin, described the study as showing that “vast regions of the
Totten Glacier in East Antarctica are fundamentally unstable.”

Indeed,
the Totten Glacier watch has been ramping up lately: Scientists have
already documented that warm ocean waters can reach the glacier’s
base and that the enormous ice shelf that currently stabilizes it,
extending over the top of the ocean, is melting from below. The
glacier is thinning quickly, and its grounding line, where the ice
shelf descends and meets the seafloor, has retreated inland three
kilometers between 1996 and 2013 in some areas.

Finally,
recent research has suggested that Totten can only lose a tiny 4.2
percent of its remaining ice shelf before the structure starts losing
the ability to brace the larger glacier, holding it in place. It all
points to a region of enormous vulnerability, and one that is already
undergoing change.

“In
a warming world, West Antarctica and regions of East Antarctica that
are below sea level will be the most likely to change,” says Robin
Bell, an Antarctic expert with the Lamont-Doherty Earth Observatory
at Columbia University, who reviewed the new study for The Post.

In
the new research, researchers took aircraft-based measurements across
the vastness of Totten Glacier, and the extremely deep and thick ice
canyons behind it — which scientists call “subglacial basins” —
in order to understand a critical yet invisible feature: precisely
what the layers of rock beneath the ice are really like.

This,
in turn, provides a clue to the behavior of this region in past warm
eras. When marine-based glaciers move back and forth across a
seascape repeatedly, they grind against the seafloor and dig up piles
of looser sediment, such as sandstone, depositing them in a new
location. But when glaciers move more quickly, sediment beneath them
remains more undisturbed.

The
radar, magnetic and gravity measurements conducted in the study found
key regions where Totten Glacier and the connected systems of ice
behind it lie atop plenty of sediment — suggesting the glacier
retreats rapidly in these areas. But it also detected areas where
there isn’t much sediment at all, suggesting that it grinds away in
these locations a great deal, or in the words of UT-Austin’s Jamin
Greenbaum, one of the researchers behind the study, is able to
“ping-pong back and forth” over long time periods.

The
gist is that while Totten may be in a relatively stable configuration
now, if it retreats far enough, then it can start an unstable
backslide into deep undersea basins and unload a great deal of ice,
raising seas first by close to a meter and then considerably more
than that.

“There’s
multiple stages. But at each stage, we see a bigger contribution to
sea level rise and a bigger proportion of contribution to sea level
rise from this system. This system keeps going, and its role keeps
increasing, as we get to bigger and bigger amounts of sea level
rise,” Aitken says.

Scientists
believe that Totten Glacier has collapsed, and ice has retreated deep
into the inland Sabrina and Aurora subglacial basins, numerous times
since the original formation of the Antarctic ice sheet over 30
million years ago. In particular, they believe one of these retreats
could have happened during the middle Pliocene epoch, some 3 million
years ago, when seas are believed to have been 10 or more meters
higher (over 30 feet) than they are now.

“This
paper presents solid evidence that there has been rapid retreat here
in the past, in fact, throughout the history of the ice sheet,”
Greenbaum says. “And because of that, we can say it’s likely to
happen again in the future, and there will be substantial sea level
implications if it happens again.”

That
said, the research suggests that the Totten system presents a very
complex landscape and that the ice will have to surmount numerous
different hurdles before all of it is able to empty into the ocean.

First,
there’s the current ice shelf and a marine-based area that extends
back about 150 kilometers inland. Much of this area is very deep —
with ice grounded over a kilometer below sea level — but it also
shallows somewhat as you move inland, rising into a ridge whose peak
is often just 200 meters below sea level, though it also contains
much deeper channels.

The
good news is that retreating up a ridge is harder for ice to do. The
bad news is that the retreat has already begun in this area. If the
ice sheet manages to retreat back over this region — which it could
do if it loses its ice shelf entirely, Aitken says — then seas
could increase 90 centimeters, or close to a meter.

Assuming
Totten clears this region, though, 90 centimeters would be just the
beginning. After that, there is a large area where it seems that the
ice sheet can retreat very rapidly. That’s because there is a
plunge downhill in this area — dubbed the Sabrina Subglacial Basin
— and the ice is not stable. Traversing it would raise the total to
more than 2 meters of sea level rise, as the ice would retreat
backward several hundred more kilometers.

“Once
you’re over that hill, you get this big runaway retreat into the
interior of the continent, which gives you a very substantial amount
of sea level rise,” Aitken says.

Then,
at the back of this area, the ice sheet would take on a new and more
stable configuration again, says Martin Siegert, a glaciologist with
Imperial College London and one of the study’s authors. It would
feature relatively shallow and stable areas cut into by deep fjords,
or subsea valleys, that are as much as 1,000 meters deep. In these
vulnerable fjords is where ice loss would occur, just as it does
today at Greenland glaciers like Jakobshavn and Helheim.

“When
the ice sheet starts to retreat, it will go back quickly toward those
fjords there,” Siegert says. “So it will go back to a Greenland
style type ice sheet.”

Once
again, this area would prove harder for the ice to move across. But
if it does, then all bets are off — the ice front would plunge down
into the extremely deep Aurora Subglacial Basin, and total sea level
rise could reach 4 meters, or over 13 feet (on top of major
contributions from other parts of Antarctica, which would also surely
have retreated at this point).

In
light of this research, the key questions become: How rapidly Totten
can pull off these various retreats? And how much warming of the
atmosphere, and the ocean, would be required to push it into a motion
even greater than what it has seen so far?